To the OP, I can appreciate that your friend is willing to question where he gets his knowledge from (his epistemology).
I think that's the core bit of it. For thousands of years, people thought the Earth was flat, the center of the solar system, and the center of the universe. They believed in humors, gods, 4 elements and whatever else.
Just because everybody tells you something doesn't make it true. This is especially true when 'everybody' is telling you other things you don't believe. Take for instance the entire societies with their own newspapers and lists of facts that everybody 'knows' to be true. Why would you believe some foreigner with a list of names, when your own (personage of authority) tells you they are lying to you, especially when they are using it as justification to act against your interests and for some other group?
I'll also weigh in on 'denying isn't innately evil' with the caveat that 99 times out of 100 or better it's used to further evil ends.
Secondly, be careful about your implied value judgements. Genius doesn't imply good, it just implies intelligence and skill at reaching a desired output. It's how you use it that gives it moral alignment.
Thirdly, be careful not to equate any measurable human quality with humanity and worth. There are a lot of people scared to death that if we admit that left handed people are smarter, nicer, more likely to have a full head of hair, or are better atheletes will immediately serve as a justification to use policy to elevate or discrimenate against them. That's scary, because it leaves our moral standpoint in the arms of future science. You can say that two peoples worth is the same, but if you start equating that to different groups measurable equality, you're leaving the door open for future science to justify future eugenics. The answer isn't to say A = B because A MUST equal B for great justice, it's to say that A and B aren't even a factor.
TL\DR : Nobody has or is even capable of 'Proving' the holocaust in this forum or over the internet. The only way to prove is to see the records and to carbon date them yourself, and see bodies and carbon date them and whatnot. There is in fact, no way to 'prove' it beyond going back to perhaps 50 original sources. Everything has spread from that.
*Not saying I don't believe...